Circulatory modification in the blue crab Callinectes sapidus, during exposure and acclimation to low salinity

A pulsed-Doppler flowmeter was used to measure heart rate and haemolymph flow rates in each arterial system of the blue crab, Callinectes sapidus, enabling calculation of stroke volume and cardiac output. During exposure to a 6–6–12 h salinity cycle of 100–25–100% seawater, there was an immediate increase in heart rate upon dilution of the medium. After this initial increase it decreased steadily, but remained elevated above levels in 100% seawater. A smaller increase in heart rate occurred when the salinity was raised, declining thereafter and reaching pre-treatment levels after 6 h in 100% SW. There was a slight decrease in stroke volume of the heart, but overall this resulted in an increase in cardiac output when the salinity was lowered. Differential haemolymph flow through each major arterial system also occurred. There was an increase in flow rates through the anterior aorta, anterolateral arteries and sternal artery during the first 2 h of low salinity exposure and smaller increases occurred again when the salinity was raised to 100% seawater. No significant changes in flow were observed in the hepatic arteries or posterior aorta. During a 72 h acclimation period in low salinity, similar increases in cardiac parameters and flow rates were observed in the first 6 h. These values declined to levels comparable to those in 100% seawater, after 40–50 h acclimation in low salinity. The changes in cardiovascular parameters are not directly related to the osmoregulatory physiology of this species, but appear to be due to specific behaviours occurring in response to low salinity. The results obtained here for this efficient osmoregulator are compared and contrasted with similar studies on Cancer magister, which is classified as a weak hyperosmoregulator.